The blood glucose level is an important marker for diabetes. As for an examination for diabetes, other than a clinical examination performed in a hospital laboratory or the like, a simple determination (point-of-care testing (POCT)) such as a simple examination by a medical staff member or the like or a self-examination by a patient himself or herself is performed.
This simple determination is performed using a glucose diagnostic kit or a determination device (POCT device) such as a biosensor, and in such a POCT device, conventionally a glucose oxidase has been used. However, such a glucose oxidase is affected by a dissolved oxygen concentration and an error in the measured value is caused. Therefore, it is recommended to use of a glucose dehydrogenase which is not affected by oxygen.
Examples of the glucose dehydrogenase include a coenzyme-unconjugated glucose dehydrogenase which requires nicotinamide adenine dinucleotide (NAD) or nicotinamide adenine dinucleotide phosphate (NADP) as a coenzyme and a coenzyme-conjugated glucose dehydrogenase which requires pyrroloquinoline quinone (PQQ), flavin adenine dinucleotide (FAD) or the like as a coenzyme. Among these, the coenzyme-conjugated glucose dehydrogenase has advantages that the enzyme is less likely to be affected by impurities as compared with the coenzyme-unconjugated glucose dehydrogenase, the determination sensitivity is high, and further, in principle, the POCT device can be produced at low cost.
However, a conventional PQQ-conjugated glucose dehydrogenase has low stability and also has a disadvantage that it reacts also with maltose and galactose. Maltose is a sugar used in an infusion, and when the PQQ-conjugated glucose dehydrogenase reacts with maltose, a blood glucose POCT device displays a higher blood glucose level than the actual value. Due to this, a patient administers an unnecessary insulin injection to the patient himself or herself, resulting in the occurrence of a hypoglycemic event such as impaired consciousness or comatose states, which has been a big problem.
In particular, as for the current use of the blood glucose POCT device, not only it is used for simply determining the blood glucose, but importance as a means for self-care and self-treatment by a patient is increasing and the use of a self-monitoring of blood glucose (SMBG) device to be used for the purpose at home is expanding. Therefore, the demand for determination accuracy is considered to be very high.
In fact, an official notice to draw attention about the use of a blood glucose meter using an enzyme requiring PQQ as a coenzyme was issued from the Ministry of Health, Labour and Welfare in Japan in February 2005 to patients under administration of maltose infusion or dialysate containing icodextrin (Pharmaceutical and Food Safety Bureau Notice No. 0207005 issued on Feb. 7, 2005, etc.).
On the other hand, as the coenzyme-conjugated glucose dehydrogenase which catalyzes the dehydrogenation reaction of glucose and requires FAD as a coenzyme, an Agrobacterium tumefaciens-derived enzyme (J. Biol. Chem. (1967) 242: 3665-3672), a Cytophaga marinoflava-derived enzyme (Appl. Biochem. Biotechnol. (1996) 56: 301-310), a Halomonas sp. α-15-derived enzyme (Enzyme Microb. Technol. (1998) 22: 269-274), an Agaricus bisporus-derived enzyme (Arch. Microbiol. (1997) 167: 119-125, Appl. Microbiol. Biotechnol. (1999) 51: 58-64), and a Macrolepiota rhacodes-derived enzyme (Arch. Microbiol. (2001) 176: 178-186) have been reported. However, these enzymes oxidize a hydroxy group at the 2- and/or 3-position of glucose, have a high activity for maltose, and have a low selectivity for glucose. Further, a coenzyme-conjugated glucose dehydrogenase derived from Burkholderia cepacia having a high activity for maltose in the same manner is also known. However, an original naturally occurring enzyme is a heterooligomer enzyme comprising three kinds of subunits: α, β, and γ, and is known as a membrane-bound enzyme. Therefore, there are problems that a lysis treatment is required for obtaining this enzyme, simultaneous cloning of a necessary subunit is required for exhibiting a sufficient activity by cloning, and so on.
On the other hand, the present inventors have purified a novel soluble coenzyme-conjugated glucose dehydrogenase which requires FAD as a coenzyme and is not a membrane-bound type from Aspergillus terreus (Patent document 1). This coenzyme-conjugated glucose dehydrogenase described in Patent document 1 has unprecedented excellent properties that it oxidizes a hydroxy group at the 1-position of glucose, has excellent substrate (glucose) recognition performance, is not affected by dissolved oxygen, and also has a low activity for maltose (the activity for maltose is 5% or less and the activity for galactose is also 5% or less with the activity for glucose taken as 100%).
However, the coenzyme-conjugated glucose dehydrogenase described in Patent document 1 is isolated and extracted from a liquid culture of a wild-type microorganism (such as a microorganism belonging to the genus Aspergillus), and the production amount thereof is limited. Besides the fact that the production amount of the enzyme is extremely small, a large amount of sugars are linked to the enzyme, and the enzyme is in the form covered with sugars which are different from N-linked or O-linked sugar chains bound to a common enzyme (which might be called “a sugar-embedded enzyme”). Therefore, the activity of the enzyme is difficult to detect (the enzymatic activity is low), the sugar chains cannot be enzymatically or chemically removed, and as a result, in electrophoresis, almost no staining is achieved by common protein staining (coomassie brilliant blue G-250 or the like), and also it is difficult to read amino terminal and internal amino acid sequences of the enzyme which provide information necessary for acquiring a gene from the enzyme subjected to a common purification procedure. Accordingly, it is not publicly known that the cloning of a gene of this enzyme was successful or the expression of the activity of this enzyme was confirmed.
On the other hand, the existence of a coenzyme-conjugated glucose dehydrogenase derived from Aspergillus oryzae was suggested in 1967 (Non-patent document 1). However, only partial enzymatic properties were revealed, and although a property that the enzyme does not act on maltose was suggested, there has been no detailed report with respect to the coenzyme-conjugated glucose dehydrogenase derived from Aspergillus oryzae since then, and also there has been no subsequent report with respect to a coenzyme-conjugated glucose dehydrogenase derived from other microorganisms or an enzyme which oxidizes a hydroxy group at the 1-position of glucose, and also no report with respect to the amino acid sequence or gene of the coenzyme-conjugated glucose dehydrogenase has been found at all.
Further, an idea of using a glucose dehydrogenase EC 1. 1. 99. 10 in glucose determination (see Patent document 2) is known, however, an FAD-conjugated glucose dehydrogenase has not been produced at a practical level, and the enzyme has not been actually used in a sensor or put into a practical use. The reason is considered that the activity of this enzyme in microbial cells was very low, and even if the enzyme was secreted to the outside of microbial cells, the amount thereof was very small, and moreover, the enzyme was covered with a large amount of sugars, and therefore the activity was low, and even the detection thereof was difficult, and thus the gene thereof could not be cloned.    Patent document 1: WO 2004/058958    Patent document 2: JP-A-59-25700    Non-patent document 1: Biochem. Biophys. Acta., 139, 277-293, 1967